American suburban front gardens, like their British counterparts, tend to be notoriously conformist. But drive down Skidmore Avenue in Portland, Oregon, and a surprise awaits you. The garden is densely planted, but the plants are mixed in with broken pottery - mostly plant pots, some complete enough to have plants in them, others so broken they're more of a mulch.

The house number, 4527, is painted on four pots that take up most of a garden seat. Anyone whose curiosity gets them out of their car will soon discover that parts of the paving leading through the garden have been made into a mosaic - mostly pebbles, but also including old tools, taps, cutlery, even salt and pepper shakers. Welcome to "Nancyland", as Nancy Goldman calls her garden - and welcome to a style best described as "junk gardening".

Most of us tend to think of the garden as basically about plants, while some view it as more of an outside room to be landscaped. Junk gardeners, however, are more likely to think of it as an art gallery or open-air studio. But why is it we see so little of this style of gardening in Britain? Perhaps we take our gardens a bit too seriously?

The best junk gardens are to be found on North America's west coast. Most are made by hobby gardeners (Goldman's day job is as a college administrator); others by artists. Among the latter is Marcia Donahue of Berkeley, California. Visitors to her home and small suburban garden are greeted by bamboos and acanthus mulched with old bowling balls. Her densely planted back garden is full of assemblages of objects, often with a distinctly shrine-like quality to them, a bit like the votive offerings sometimes seen around holy wells or other sacred places. "For me," says Donahue, "gardening is a sculptural activity. When I create these little shrines in the house or garden, it's a way of organising things. They're also a universal expression of humanity."

For Goldman, meanwhile, collecting and arranging artefacts is about "keeping the garden interesting and ever-changing. There is a sense of whimsy and friendliness." She started doing this 10 years ago. "I've always been a bit of a magpie, and I started using artefacts to accent the plants. I don't like holes in the garden, so I'd fill gaps."

Some of her collections serve a practical purpose, such as the old push-pull mowers she gathered together in one area (which she dubbed Mowtown). "They are useful because you can grow stuff up them or use them as supports."

Of course, both Goldman and Donahue like to hang out at second-hand stores and architectural salvage yards, and enjoy scavenging in skips. "I see stuff and I respond to it," Goldman says. "I never know what I am going to find, which is part of the fun."

Making connections between things is important, too: Donahue says a collection of flying crockery "is all about gardening being soul food", while Goldman says she likes to "take something and give it a tweak, like a pun or play on words. Like the old trowels I used to edge a bit of the lawn."

Collecting artefacts sometimes leads to making new ones to complete a piece. One of the high points of Goldman's garden is a framework she made to support a grapevine, using wooden T-supports and a steel reinforcing bar. It didn't feel finished, "so I got a friend to make some pretend clothes in sheet steel, and hang them up like it was a washing line."

Goldman and Donahue are both "proper" gardeners, too. Donahue is involved in a local garden club and Goldman is president of the Hardy Plant Society of Oregon. A big part of the reason their gardens work so well is that the planting is diverse and well-chosen, and there is often an interplay between the sculptures and plants: grasses and coloured foliage, in particular, look good next to metals and painted surfaces. Above all, here are two creative people demonstrating the two most important things about gardening: be true to yourself and have fun.

Cool or kitsch?

Here are some tips on how to have a junk garden, not a junkyard:

Be disciplined Collect things that have a theme or are related to each other. Suitable themes might be defined by the material they're made from - terracotta, say - their function, their colour, shape or size.

Find your locations Identify places in the garden that need something extra; they can then be transformed from dead spots to focal points.

Combine junk with good planting Effective junk gardens are well-planted and looked after - the junk is very much an enhancement. Interesting, varied foliage is particularly effective, as are many smaller evergreen grasses with coloured foliage: carex, luzula or Stipa arundinacea, which isn't evergreen but goes a wonderful russet in winter. Compact evergreen shrubs can be good, too: box, hebes, Euonymus fortunei, Japanese azaleas and sarcococca. The 'wire-netting bush', Corokia cotoneaster, is a real winner.

Create groups of objects This will also look more intentional. Objects should relate to their surroundings, which is why using old garden tools is a good, and safe, way to start.

Try to create surprises This could be objects you don't expect to see.

Beware of being ironic Others may read it as the real thing, so go careful with those gnomes.

Make connections that get people thinking What are those old fruit bowls doing under that apple tree?